What are Acid Sulphate Soils?
Acid sulphate soils are a mix of low-lying coastal clays and sands that contain sulfur bearing compounds at concentrations above 0.05% in clays and 0.01% in sands. Such soils are formed as naturally occurring sediments are deposited under estuarine conditions. Thus, the distribution of these soils commonly reflects the past and present distribution of coastal wetlands. The sulfur bearing materials (generally in the form of pyrite) in the soil are the by-products of the bacterial breakdown of wetland vegetation using sulphate compounds from seawater in the absence of oxygen.
Potential acid sulphate soils are generally found less than 5 metres above sea level. When exposed to oxygen as a result of drainage or disturbance, these soils can produce sulfuric acid, releasing toxic quantities of iron, aluminum and other metals. These in turn can leach through the soil affecting groundwater and surface waters, thus affecting urban areas, farm productivity and marine life. This is a major issue along the coastal fringe of NSW.
How are they Harmful?
Acid sulphate soils are harmful in several ways:
- Loss of vegetation. The sulfuric acid that is produced upon the oxidisation of pyrite lowers the pH in the soil to extreme levels, at times less than 3.0pH. This has significant implications for farming and vegetation survival.
- Ecological impacts on the aquatic system. Following high discharges of toxins, large instance of aquatic loss of life can occur. Furthermore, habitat degradation, outbreaks of disease and biological defects are also common.
- Economic costs. Engineers must cope with extremely aggressive soil conditions for construction when using concrete and steel in acid sulphate soils. Corrosion by acidity and salts can cause serious problems for pipelines, pylons, piles and other structures. Significant costs can be incurred to the community through maintenance of public structures such as bridges, pipelines and floodgates.
What can be done?
Planners and developers must be aware of the presence of acid sulphate soils when preparing land development proposals. The engineering hazards and environmental consequences associated with acid sulphate soils must be taken into account during initial design of the development. Large costs have been, and will continue to be, incurred through ignorance of the problem.
Thus, in order to determine the acidity of the soil before construction, it is helpful to conduct an Acid Sulphate Soils Assessment. The objectives of an ASSA investigation are to:
- Establish whether acid sulphate soils are present on the site; and
- Determine if concentrations warrant the preparation of an acid sulphate soils management plan.
Once this has been conducted, an appropriate remediation plan is able to be created as appropriate. Thus, as acid sulphate soils are found in every coastal floodplain of NSW, if you are considering building or renovating within these areas, it is essential that you assess the state of your soil before commencing to build. Otherwise, you could potentially be up for large repair costs due to the corrosion of building materials.
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A. D. Envirotech Australia Pty Ltd (ADE)
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